It is technically 4.4% due to level difference. The reason we use 5% and 102.4% is that we are basing it off of our level. Use souldrinker, swap as many gems from pure mastery to hybrid as you can while maintaining 102.4%

We live in a society where people born on third base constantly try to steal second, yet we expect people born with two strikes against them to hit a homerun on the first pitch.

I personally try to be around 99% unbuffed, as I eat the normal feast and use Mastery/Resistance Elixir to get a bit over the target 102.4%. You can go even lower if you want to bring your own Mastery food, but this leaves a larger window if you do die and get BR during a fight.

You're probably fine to swap in Mastery/Stam gems in your chest and pick up a free 30 stamina; there is a Greater Mastery to glove enchant that will give you +15 mastery there. If you're still short, you can pick up some CTC by reforging your rings.

Your trinket selection is probably fine; running dual-mastery trinkets is overkill in T13 unless you want to replace all of your yellow gems.

What if I just really really love the on use abilities of the mastery trinkets and reforge off the mastery

I love stay of execution, dodge from fire of the deep, and obviously mirror of broken images! Plus I could put all blue gems in my stuff if I'd just leave em both un reforged, but I'm still an avoidance whore ( reforging to threat stats shortly)

Ahh, right, thanks for clearing that up! I didn't think the boss reduction was applied to the miss portion for these numbers somehow. So for the purposes of 102,4%, 5% is indeed the correct number, and looking at your character sheet you want dodge + parry + block = 97,4%?

yes, for a shield tank, you can just add up character sheet Dodge + Parry + Block = 97.4% to account for 5% miss and the extra 2.4% chance the boss has against all of your defenses.

Its best to add them up when you are raid buffed, assuming you use the same buffs & gear for all fights. Sometimes I still swap in the TB Mirror and so lose about 60 Mastery vs my regular trinket, which I can then make up with Mastery food over the dodge based feast if I need to.